Field of the Invention
The invention lies in the field of internal combustion engines. More specifically, the invention relates to a method of controlling the exhaust gas recirculation in an internal combustion engine in which, dependent on a load variable and other input variables, in particular the engine speed, a setpoint value, stored in memory in a control unit of the exhaust gas recirculation rate is ascertained and a control signal for controlling an exhaust gas recirculation valve is derived from the setpoint value for the exhaust gas recirculation rate. The setpoint value stored in the control unit is appropriate for steady-state operation of the engine.
In internal combustion engines, especially diesel engines, exhaust gas recirculation is employed to reduce the proportion of NO.sub.x in the exhaust gas. From a technical combustion standpoint, it is necessary to furnish defined exhaust gas recirculation rates over the entire engine operating range. The exhaust gas recirculation rate is dependent on the difference between the charge-air-pressure in the intake pipe and the exhaust gas back pressure, as well as on the setting of an exhaust gas recirculation valve that is located in a connection between the exhaust gas system and the intake conduit. German published patent application DE 195 02 368 A1 describes a method for determining the actual recirculation rate.
To assure a defined exhaust gas recirculation rate, open-or closed-loop control devices are known. For instance, German Patent DE 42 22 414 C2 describes an exhaust gas recirculation control device for an internal combustion engine. As a rule, controlling the exhaust gas recirculation is done by a control unit, which in dependence on input variables such as the intake pressure, accelerator pedal position, and engine speed determines a requisite exhaust gas recirculation information, in particular the exhaust gas recirculation rate, for operating the engine. This is conventionally done by storing setpoint values in a performance graph for various input variables. These setpoint values are ascertained with the aid of static tests performed on a test bench. The performance graph applies to steady-state engine operating phases.
For a non-steady operating state, that is, upon transitions from one steady operating state to another, performance graph specification of setpoint values is not always satisfactory, particularly with regard to the emission behavior and fuel consumption. Upon a rapid load change, such as starting up, the setpoint value rises, passing through points of the performance graph in which a higher exhaust gas recirculation rate is desired under steady-state conditions. On starting or during acceleration, however, because of the sharp rise in the fuel quantity, air deficiency phenomena occur, since the pressure, present in the steady state, in the intake system is unavailable, or not yet available, in this non-steady operating phase. The consequence of this in diesel engines is increased soot. An additionally increased exhaust gas recirculation rate reduces the oxygen content in the supplied air and thus exacerbates the problem. The closed-loop control of the EGR rate has therefore until now been turned off during idling or upon acceleration. It is difficult to define the conditions for turning the closed-loop control on and off.